This invention relates to a housing for a flammable gas detector.
One type of flammable gas detector employs a heated sensing element to oxidize any flammable gas present. Oxidation of flammable gases at the sensing element causes a change in electronic properties of the sensor which is detected to indicate the presence of a flammable gas. For example, a catalytic bead sensing element may comprise a coil of wire embedded in a porous bead containing a catalyst. The bead is heated electrically by passing current through the wire. If flammable gas is present at the bead, it oxidizes exothermally in the presence of the catalyst, causing the temperature of the bead, and hence the wire coil, to increase. The change in resistance resulting from the change in temperature is detected to indicate the presence of a flammable gas. Frequently two bead elements are used in a Wheatstone bridge arrangement with one element acting as the sensor and the other being of similar construction but rendered inert and acting as a control. Higher output, and hence greater sensitivity, can be obtained by using multiple beads connected in series or through the use of electronic amplification of the output signal.
Although, for operation of the detector, the gas within the detector housing must communicate with the surrounding ambient gases, for reasons of safety, the gas undergoing oxidation within the housing must be prevented from igniting any flammable a as outside the housing. For this purpose, known detectors include in the wall of the housing a flame arrestor in the form of a sinter element, through which the interior of the housing communicates with the outside. The flame arrestor should provide as small a resistance to diffusion as is consistent with its primary requirements of strength and quenching the flame front.
The housing for the sensor must be flameproof and capable of withstanding internal explosion without allowing a flame front to propagate outside the housing. Any joint or gaps in the housing must be small enough to attenuate the flame front such that it is incapable of igniting the gas outside the housing.
Known housings are cast from metal. All potential flame paths require 100% inspection in order for the housing to meet the relevant safety standards. Not only must the flame arrestor element be itself designed to meet the required safety standards but it must be secured to the housing in such a way as to prevent a flame path being provided at the flame arrestor element/housing interface. In order to form a satisfactory flame arresting arrangement, the housing and sinter element must be in intimate contact along the entire length of the sinter element/housing interface. The periphery of the sinter element, and the internal surface of the housing over the region of contact with the sinter element, must be precisely formed. In practice to meet the tight tolerances required for satisfactory performance, the cast metal housing has to be precision machined.
In manufacturing the known flammable gas detectors, the sinter element is fixed to the housing in a separate operation. The sinter element may be glued to the housing, or the housing may be peened over the sinter element, or the sinter element may be retained in the housing by peening the housing over the edges of the sinter element, and the sinter element subsequently glued in position.
The precision machining, quality assurance and fixing operations required to ensure that the known housings meets safety standards, makes them time-consuming and expensive to manufacture.
According to the present invention there is provided a housing for a flammable gas detector comprising a housing body with an aperture through which the interior of the housing body communicates with the outside, a gas permeable flame arrestor element located in the aperture, at least a portion of the housing body surrounding the aperture being molded from plastic materials, the portions of the housing body that form the aperture being molded around the flame arrestor element with the flame arrestor element in situ whereby the flame arrestor element is fixed to the housing body. Molding the body of the plastic housing around the flame arrestor element with the flame arrestor element in situ eliminates the operation of fixing the flame arrestor element into the housing. Moulding the plastic material directly onto the periphery of the flame arrestor element elimnates the flame path at the interface between the arrestor element and the housing. The requirements for machining the housing, and fixing the sinter element to the housing in a separate operation are eliminated reducing the overall number of manufacturing operations with subsequent savings in both manufacturing time and cost.
A suitable plastic material is used for the housing body. It should exhibit which impact strength mechanical rigidity, UV stability and flame retardant properties over an extensive temperature range. Suitable plastics might be thermoplastic, for example mineral-filled PPS (polyphenylsulphide), PBT (polybutylterepthalate), or LCP (liquid crsystal polymer such as poly(benzoate-napthoate)). Alternatively, thermosetting plastics such as DMC (dough-molding compound-polyester) might be used.
The porous nature of sinter elements allows any hot plastic which comes into contact with the sinter element to be wicked into the sinter element, reducing the gas permeability of the sinter element and, therefore, its effectiveness. When a sinter element which has a support ring around its periphery is used, the housing can be designed so that the hot plastic comes into contact only with the support ring and never into direct contact with the porous sinter element.
If the housing is to be molded around a sinter element which does not have a support ring, wicking may be reduced, or eliminated, by using a sinter element which has a greater density around its periphery where the housing and the sinter element will be joined, than in the middle. By making the periphery denser, the pore size is reduced and the molten plastic material cannot penetrate the sinter material so easily where it comes into contact with the housing.